1. Field of the Invention
The present invention relates to a reading device for the optical scanning of informations on a surface reflecting the light of a moving plate-shaped carrier.
2. Brief Description of the Background of the Invention Including Prior Art
Reading provisions, also called shortly optical scanners serve the reading of audio and/or video informations, which are stored on an audio or video plate. They are in principle also suited for the reading of other optically readable data for data processing. The principle of the storage and of the reading of optically readable data is illustrated in general in Philips Technical Review 40, 151/155, 1982, No. 6.
Usually plate-shaped rotating recording carriers are employed, such as picture storage and video plates. However, also other storage methods are applicable, as far as these light reflecting surfaces are employed for the storage of optically readable data.
The reading provision comprises substantially a scanning head with a light source, which is in general a laser light generating semiconductor, as well as an optic system, which generates one or several light spots on a carrier surface. The scanning head is further provided with means for separating the reflected light from the emitted light such as for example as a beam divider as well as with detection elements for the conversion of the detected light into electrical signals.
Since the track density is very high, the scanning head has to follow the track with high precision and in particular with a tolerance of plus-minus 0.1 micrometer of the recorded track. This accuracy cannot be reached by mechanical precision, since alone deviations based on the eccentricity of the recording carrier and of the recording apparatus amount to a multiple of the above recited tolerance range (up to 3000 micrometer).
Therefore control provisions are required, which lead the scanning head controlled by the recorded track.
It is known to provide a coarse control with larger steps for guiding the arms supporting the scanning head and a fine control with smaller steps for the guiding of the head disposed pivotably at the arm. This keeps the effective masses, which are to be moved during the fine control, small, and a rapid control is made possible.
If the displacement motion of the following scanning head surpasses a preset threshold value, then a switching from fine control to coarse control is performed.
According to a known principle, where the scanning head is maintained in a defined medium position and is moved against the restoring force of an elasticity, then the maximum value of the electric current flowing through the coil for the radial fine-drive is a measure for the threshold.
Narrow pulses occuring in an automatic control circuit of an apparatus of this kind can already surpass the threshold for the switching on of the coarse drive, even though the latter has not reached the mechanical threshold for the switching of the fine drive setting to a coarse drive setting because of the narrow pulse width and based on the mechanical inertia of the scanning head. In order to avoid this, such pulses have to be suppressed, and for example kept away from the threshold provision by way of a low pass filter. On the other hand pulses, which correspond to an actual radial error deviation of the light spot versus the track, cannot be influenced by such a low pass filter. Therefore, a compromise has to be accomplished with such therehold provision between the suppression of disturbing values and the transmission of the desired correction signals.
This compromise has not to be taken if the actual deviation of the scanning head from the set position or from a reference position is determined. In case of analog record players (for example the type SL3 or SL5 of the company Technics) the deviation of the sound pick-up is determined immediately via a light barrier arrangement, where a diaphragm is applied at the movable sound pick-up arm, which is disposed on the light path between a light source and a two segment photo detector. The light diaphragm and the photo elements are disposed such that the segments in case of a deviation of the sound pick-up are illuminated differently strong. An automatic control signal for the radial position correction of the sound pick-up can be determined from the two electrical signals of the sound pick-up element. Pulses occurring in the automatic control circuit which are not followed by a motion of the sound pick-up are not determined in contrast to the above described arrangement and therefore they cannot lead to an erroneous correction.
Such a light barrier arrangement is relatively expensive because of the required precision and is subject to disturbances because of the danger of different soiling or aging of the photoelement segments. In addition, this light barrier arrangement itself cannot be employed to produce controlled desired displacement motions of the scanning head.